This invention relates to a method for anchoring a structural member to a foundation to resist uplift forces imposed on the structural member. Specifically, this invention relates to an improved method of anchoring structural members, having interior cavities, to foundations with an anchor member embedded in the foundation and received by the internal cavity, and fastened to the structural member by fasteners.
All structures must be designed to resist lateral forces. Lateral forces on buildings are typically produced by wind loading and seismic forces. The extensive damage caused to buildings by the January 1994 earthquake at Northridge, Calif has demonstrated that systems for resisting lateral forces must be improved.
Building components that resist lateral forces are called lateral force resisting systems. Lateral forces imposed on a structure can create moment forces. These moment forces, in turn, create uplift forces on certain structural members and compression forces on others. Typically, architects design structures to resist these moment forces at the structural member in tension by anchoring the member in tension to its foundation.
In general there are three types of lateral force resisting systems used in framed buildings. These lateral force resisting system are commonly referred to as shearwalls. The first type, rigid frames, resist lateral forces by bending in the frame members. The second type, trusses or braced frames, resist lateral forces by primarily carrying the resulting tension and compression forces in diagonal members or cross braces. The third type, diaphragms, are large, flat structural units that act like deep, thin beams with the structural panel or panels of the diaphragms acting as the "web" of the beam and the chords of the diaphragms acting like the "flanges" of the beam. It is thought that shear deformation is the significant action in diaphragms. In all three types of shearwall, the vertical structural members or chords will be anchored to the foundation to resist the resulting moment forces on the shearwall.
The present invention provides an improved method of anchoring structural members against tension or uplift forces. It is particularly designed for anchoring the chords or vertical end members of shearwalls where moment forces can be pronounced. One specific application for the improved holdown connection of the present invention is the side walls framing a garage opening in areas with high seismic activity. These walls are often called garage returns. Garage return shearwalls are typically narrow (being less than 2 feet wide), but tall (generally, they are over 7 feet high) to create an opening that can accommodate two cars. Often, they must provide the entire lateral force resistance for the wall, since the garage door opening provides none. With such narrow, tall shearwalls the moment forces imposed are significant and overturning and uplift is a common failure mode.
Under generally accepted construction methods, specific vertical structural members or chords of shearwalls are anchored against uplift forces with holdowns attached to anchor bolts or straps. In the case of holdowns, they attach to the side of the chords with heavy screws, nails or bolts. The holdowns receive bolts which are embedded in the foundation below. In the case of straps, they attach to the sides of the chords and are themselves embedded in the foundation. This system works well when the vertical structural members or chords of the shearwall are solid or do not otherwise allow the chord to be centered over the anchor member; however, there is often eccentric loading on the anchor bolt or strap.
New building materials such as hollow steel tubing and construction elements formed with interior cavities are becoming available which can benefit from the present invention by having their anchoring member inserted in their interior cavity to reduce or eliminate eccentric loading.